My written German is so bad I won't risk confusing you with it. If you have this problem (no sound) you are probably confused enough. I had it on my second TV box 2 days after installation and on the other one today.
Solution: Go to settings on the TV screen (the little gear symbol).
Go to sound.
Change to "stereo"
Increase the volume using the Swisscom remote.
Sound should return.
You can change the setting back to "surround sound" if you want to and the problem will not return.